1. Field of the Invention
The present invention relates to a self-tapping screw to perform fastening while tapping an internal thread at a pilot hole formed at a counterpart.
2. Description of the Related Art
Recently, in electronic devices, communication devices, office equipment and the like being widely spread in the market, a self-tapping screw capable of eliminating a process to previously tap an internal thread at a counterpart has been often used for attaching an attaching part such as a component to the counterpart made of synthetic resin or metal.
Such a self-tapping screw has been commonly desired to obtain large fastening force as tapping an internal thread in high accuracy while possibly minimized screwing torque when fastening an attaching part with a counterpart.
In a self-tapping screw disclosed in Japanese Utility-model 3140074, especially, in a self-tapping screw of claim 1 without limitation that clearance is formed between a screw shaft circumferential face (i.e., a root face) and a pilot hole inner circumferential face, an external thread is formed to be two-step-shaped constituted with a crest portion having an acute-angled rectangular shape in section and a thread enlarging body portion below the crest portion having bilateral guide leg portions (i.e., inclined planes) being divergent to be connected to the root face with an angle larger than the crest portion over the entire external thread from the tip of a screw shaft to a part right below a screw head.
However, with the self-tapping screw having the two-step-shaped external thread, there has been a problem that initial screwing torque during screwing into a pilot hole, that is, the torque when the first part of the two-step-shaped external thread taps a two-step-shaped internal thread at the inner circumferential face of the pilot hole, is increased.
In addition, with the self-tapping screw of the abovementioned configuration, material flow is not to smooth during rolling to respective flanks of the crest portion of the external thread and to respective guide leg portions of the thread enlarging body portion. Accordingly, there has been a problem that surface roughening and burrs occur at the root face, respective flanks of the crest portion of the external thread and respective guide leg portions of the thread enlarging body portion or that cracking occurs at the crest portion of the external thread.